This squash is rich in beta carotene, with iron, vitamin C, potassium, and smaller traces of calcium, folic acid, and minute amounts of B vitamins.
When kabocha is just harvested, it is still growing. Therefore, unlike other vegetables and fruits, freshness is not as important. It should be fully matured first, in order to become flavorful, by first ripening the kabocha in a warm place (77 °F/25 °C) for 13 days to convert some of the starch to sugar. Then the kabocha is transferred to a cool place (50 °F/10 °C) and stored for about a month in order to increase carbohydrate content. In this way the just-harvested, dry, bland-tasting kabocha is transformed into a smooth, sweet kabocha. Fully ripened, succulent kabocha will have reddish-yellow flesh, a hard skin, and a dry, corky stem. It reaches the peak of ripeness about 1.5–3 months after it is harvested.